1. Field of the Invention
The present invention relates generally to apparatus for sensing the flow of a fluid, and more particularly to apparatus for detecting and measuring, by optical means, the flow of a fluid that flows slowly enough for individual drops to be detected.
2. The Prior Art
A fluid flow sensor that can detect and measure the flow of individual drops of a fluid has many applications. A primary application for such a sensor is measuring the flow of fluid through a drip chamber assembly in an intravenous ("IV") fluid infusion system. Such an IV fluid infusion system generally includes a bottle containing a fluid to be administered; a drip chamber assembly; a tube connecting the bottle to an inlet at the top of the drip chamber; a valve, associated with the tube, to control the rate of flow of the fluid; a second tube connected to an outlet at the bottom of the drip chamber; and injecting means such as a hypodermic needle or catheter connected to the second tube through which the fluid passes into the patient.
A drip chamber assembly is usually cylindrical in shape and has transparent walls enclosing a drip chamber. The fluid enters the assembly through a drop former at the top of the chamber and falls, one drop at a time, through the chamber. The flow of the fluid through the chamber can be monitored by visually observing the falling drops, but various kinds of flow sensors have been developed to detect the fall of these drops automatically and thereby free the human observer for other tasks.
Examples of fluid flow sensors that can monitor the flow of fluid through a drip chamber assembly are disclosed in U.S. Pat. No. 3,596,515, issued to Cramer on Aug. 3, 1971, and U.S. Pat. No. 4,397,648, issued to Knute on Aug. 9, 1983, both assigned to the assignee of the present application. A few sensor of the kind disclosed in these patents generally employs an optical energy emitter on one side of the drip chamber asembly and an optical energy detector on the other side. The energy emitter usually emits optical energy in the infrared or visible light spectra (wavelength between about 300 nanometers and 2 microns). This optical energy passes through the transparent walls of the drip chamber assembly and then strikes the detector. As each individual drop falls through the chamber, it interrupts the flow of optical energy, causing the detector to produce a corresponding output signal. This output signal is then applied to a monitoring device for further processing. The monitoring device typically either sounds an alarm if the fluid stops flowing or monitors the rate of flow of the fluid.
Existing flow sensors such as those disclosed in the cited patents have certain shortcomings. One of these is the need accurately to align the energy emitter and detector during manufacture, a procedure that adds significantly to the overall cost of making flow sensors. It would be possible to manufacture flow sensors more economically if this step could be simplified or eliminated.
Also, the detector must be shielded so that it will not be affected by ambient optical energy, and all of the parts in the optical path must be kept clean to prevent a reduction in sensitivity that can result from dirt or moisture interfering with the passage of optical energy. A sensor that is less sensitive to ambient energy than existing sensors, and that is easier to clean, would be more reliable and easier to use in the field than existing sensors.
In addition, the transparent walls of the drip chamber assembly refract the optical energy as it passes through. This refraction renders the detector unable to detect drops of fluid that do not pass near the center of the chamber as they fall. If the chamber is kept in a nearly vertical orientation, the detector will be able to detect the drops without difficulty because they will all fall through the center of the chamber, but if the chamber is tilted more than about 15 degrees away from a vertical orientation, the drops will tend not to fall through the center of the chamber and the detector will be unable to detect them. A flow sensor that could function properly when used with a tilted drip chamber as well as with a chamber that is constrained in a vertical orientation would be more versatile, would be easier to use, and would be less likely to give a false "not flowing" alarm than existing flow sensors.
The use of IV systems as essential elements of modern medical care is continuing to expand, and with this expanding use has come a growing demand for IV systems that can function with a minimum of human monitoring. This demand has in turn led to a need for a fluid flow sensor that can be more economically manufactured than existing sensors, that is relatively unaffected by ambient optical energy or by dust and moisture in the environment, and that can detect the flow of fluid through a drip chamber even if the chamber is tilted far from its vertical axis.
One suggested approach to this problem is to employ a non-standard drip chamber assembly having walls formed in the shape of special lenses that can focus rays of light through the drip chamber and onto a photosensitive chip of material located adjacent thereto. However, such drip chamber assemblies are relatively costly to manufacture, and a fluid flow sensor adapted for use with them cannot be used with conventional cylindrical drip chamber assemblies. Unlike a conventional cylindrical assembly, the lens-shaped walls of such a non-standard drip chamber are rigid, making it more difficult to prime the system. In addition, this approach does not address the problems caused by noise, dirt and moisture in the environment.
It will be apparent from the foregoing that there is a need for an optical fluid flow sensor that can be used with conventional cylindrically-shaped drip chamber assemblies, that can be manufactured more economically than can existing sensors, that is relatively insensitive to ambient energy and that is easy to keep clean in the field, and that can detect the flow of fluid even if the drip chamber assembly is tilted far from a vertical orientation. The present invention satisfies this need.